The present invention relates to a photonic switch, and in particular to an apparatus and a method suitable for testing the operation of a photonic switch.
Communications networks are increasingly becoming all optical networks, incorporating photonic (optical) switching. Photonic switches are typically fabricated using Micro Electro-Mechanical Structures (MEMS) technology. A recently developed photonic switch of this type is described in xe2x80x9cFree-Space Micro Machined Optical Switches for Optical Networkingxe2x80x9d by LY Lin et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 No. 1, January/February 1999; which is incorporated herein by reference. Such switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. MEMS switches typically use moveable mirrors to re-direct optical paths within the switch in order to complete an optical signal or channel connection across the switch.
FIG. 1 shows a schematic diagram of a typical MEMS photonic switch 100. The switch 100 is bidirectional, but for simplicity is assumed to comprise 4 inputs in the form of optical fibres 112, 114, 116 and 118, and 4 outputs which are also optical fibres 122, 124, 126 and 128. Each input and output has an associated lens 104 which collimates the beam from each input and focuses the respective beam at each output. Such a switch is generically referred to as a 4xc3x974 switch (number of inputs x number of outputs).
The switch 100 is a cross point switch, having a switching device (a mirror, 106) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch 100 thus has a four by four array of mirrors 106 mounted on a surface 102.
In this particular switch, each mirror may be moved between two stable positions. FIGS. 2a and 2b illustrate these positions. FIG. 2a shows the mirror in the inactivated position 106a, where the mirror is flat i.e. substantially parallel to the surface 102. FIG. 2b shows the mirror having been raised to the activated or upright position 106b, substantially perpendicular to the surface 102. This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges 108. The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating 107 such as gold. In the inactivated state, it is typical for the relatively non reflective surface 109 of the mirror to lie adjacent to the surface 102, so that the reflective coating 107 does not contact the surface 102.
FIG. 1 shows a typical operation of the switch 100. By raising the appropriate mirrors, an optical signal from each of the inputs 112, 114, 116 and 118 is directed to a respective output 128, 126, 122 and 124. For instance, an optical signal originating from input fibre 112 is formed into a collimated beam 132 by lens 104. The beam 132 then reflects off the front reflective surface 107 of a raised mirror 106b into a further lens 104 which focuses the beam 132 into the output fibre 128. It will be appreciated that by appropriate control of the array of mirrors 106, any one of the signals originating from the inputs 112, 114, 116 and 118 can be switched into any one of the outputs, 122, 124,126 and 128.
Various solutions have been proposed to test the mirror status or switch connection, in order to verify that the mirrors 106 are functioning correctly and are not, for example, jammed in either the raised 106b or flat 106a position.
One solution is to inject different optical test signals into each input port (i.e. 112, 114, 116, 118) to the switch 100 via fibre tap couplers (not shown). Such test signals would be distinct from the normal optical signal being switched e.g. of different wave length and/or modulation characteristics. Each output port (i.e. 122, 124, 126, 128) would then be connected to a further tap coupler, in order that the test signals could be extracted, detected and analysed for verification that the desired input to output connections exist. This solution is true connectivity verification. However, due to the number of components required, it would be both bulky and expensive. For instance, in a Nxc3x97N switch (where N is an integer) the required components would include 2N couplers, N sources, N detectors, as well as numerous splices and fibre interfaces; additionally there would be the assembly cost.
An alternative solution is to use electrical parameters (e.g. capacitance, inductance or resistance) to monitor the physical position of the mirrors. However, this would double the number of electrical connections to the switch matrix, and is hence impractical for large arrays of mirrors.
Co-pending U.S. application Ser. No. 09/545,545, xe2x80x9cTesting Operation of a Photonic Switchxe2x80x9d, by the same inventor, describes a method of utilising test optical signals in the plane of the switching mirrors, reflected from the rear of one or more of the mirrors, in order to test whether the mirrors are functioning correctly. This approach has limitations in that it requires the rear of the mirrors to be reflective, and requires an array of mirrors to be in certain predefined configurations for any given mirror to be tested. Simultaneous testing of all mirrors is hence not possible.
The present invention aims to address one or more of the problems of the prior art.
In a first aspect, the present invention provides a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane.
In a further aspect, the present invention provides a telecommunications system comprising a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical path defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane.
In another aspect, the present invention provides a method of testing the status of a photonic switch, the switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the method comprising the steps of providing a test optical signal incident said reflective surface, the optical path of the test signal not lying within said first plane, and measuring a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement.
Preferably, the method further comprises the step of providing an actuating signal sufficient to move the switching means such that the intensity of the reflected test signal at the predetermined measuring location would be altered, without moving the switching means sufficiently to redirect the incident optical signal. This allows the switching means (normally mirrors) to undergo a small movement to check whether the mirror is stuck, but without the movement being large enough so as to redirect the incident optical signal. If the mirrors move in response to the actuating signal, then this can be detected by the change in reflected test signal incident on the detector. This test allows re-routing to an alternative path if the mirror is fixed in position.
In a further aspect, the present invention provides a computer program arranged to perform a method of testing the status of a photonic switch, the switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the method comprising the steps of providing a test optical signal incident said reflective surface, the optical path of the test signal not lying within said first plane, and measuring a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement.